…up in animal category (4)
I believe the answer is:
clam
'up in animal category' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'up in animal category' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'malc' is hidden within 'animal category'.
'malc' backwards is 'CLAM'.
Can you help me to learn more?
(Other definitions for clam that I've seen before include "Large bivalve mollusc" , "Mollusc with tightly-closed shell" , "Shellfish - secretive type" , "Edible shellfish" , "Tightly closed shellfish" .)